(i) Field of the Invention
The present invention relates to a process for producing a polypropylene resin composition which has excellent mechanical strength and stiffness and good dimensional stability and moldability and which is compositely reinforced by a glass fiber and a lamellar inorganic filler.
(ii) Description of the Prior Art
A polypropylene resin composition reinforced with a glass fiber is excellent in chemical resistance and has a higher reinforcing effect as compared with compositions reinforced with another particle filler or lamellar inorganic filler. In the polypropylene resin composition containing 10% by weight or more of the glass fiber, strength and stiffness are particularly high, and so this kind of polypropylene resin composition is used in many fields as a useful industrial material. However, the polypropylene resin composition reinforced with only glass fiber has the drawback that molded articles obtained therefrom have large warpage deformation.
In order to remove this drawback, Japanese Patent Publication No. 64-11218 and Japanese Patent Application laid-open No. 58-206659 disclose the following three processes (1) to (3) which comprise quantitatively feeding a glass fiber and a lamellar inorganic filler to the polypropylene, melting, kneading and extruding it to compositely reinforce it.
(1) A process which comprises first melting, kneading and extruding a mixture of a polypropylene, an organic peroxide and an unsaturated organic acid to obtain the polypropylene (hereinafter abbreviated to "modified PP" on occasion) on which the unsaturated organic acid is grafted, mixing this modified PP with predetermined amounts of a lamellar inorganic filler and a glass fiber, and then melting, kneading and extruding the mixture again.
(2) A process which comprises feeding a mixture of a modified PP and a lamellar filler to an extruder through the first feed inlet of the extruder on an upstream side, and then melting, kneading and extruding the mixture while a glass fiber is fed to the extruder through the second feed inlet of the extruder on a downstream side. The extruder for use in this process is equipped with the second feed inlet through which the other raw materials are fed to a position where the modified PP can be sufficiently melted.
(3) A process which comprises quantitatively feeding the modified PP alone through the first feed inlet on the upstream side of an extruder, and then melting, kneading and extruding the mixture, while a glass fiber and a lamellar inorganic filler are fed through the second feed inlet of the extruder on the downstream side.
However, in the process in which the mixture of the modified PP, the glass fiber and the lamellar inorganic filler or the mixture of the modified PP and the lamellar inorganic filler is fed through one feed inlet as in the above-mentioned processes (1) and (2), the viscosity of the resin noticeably increases in the melting step of the resin, so that a screw in the extruder is extraordinarily worn, with the result that the continuous production is practically impossible. In addition, bridges are formed in the vicinity of the feed inlet, and the composition of the product changes by classification, and in consequence, the stability of the production is very poor.
Furthermore, in the above-mentioned process (3), there is no problem, when small amounts of the glass fiber and the lamellar inorganic filler are used. However, when both the raw materials are fed in large quantities, the bridges are formed in the vicinity of the feed inlets. In consequence, the precision of the quantitative feed lowers, so that surging and the breakage of strands often take place. In particular, it is substantially impossible to continuously produce a polypropylene resin composition containing 10% by weight or more of the glass fiber and 20% by weight or more of the lamellar inorganic filler.
In all of the above-mentioned processes (1), (2) and (3), the composition change noticeably occurs owing to classification in a hopper of the extruder, so that the product having non-uniform composition can be merely obtained. Particularly, in the case of the process (1), the strength of the product significantly deteriorates owing to the breakage of the glass fibers.
Next, reference will be made to a relation between the melt flowability and the dimensional stability of the polypropylene resin composition compositely reinforced with the glass fiber and the lamellar inorganic filler.
In general, when the glass fiber and the lamellar inorganic filler are used at high concentrations in the polypropylene, the melt viscosity of the composition increases, and the flowability, i.e., melt flow rate (hereinafter abbreviated to "MFR" on occasion) of the composition noticeably decreases. When the composition having MFR of less than 10 g/10 minutes is injection-molded, the pressure in a mold lowers at the time of the molding, even if a fairly large injection pressure is applied. In consequence, the moldability of the composition deteriorates, with the result that the effect of warpage inhibition is outstandingly impaired.
Furthermore, as a result of the extreme deterioration of the flowability, the resin does not flow to the edges of the mold and is very poor in moldability, unless the injection pressure and the molding temperature in the molding step are increased.
In order to prevent this deterioration of the flowability, it is required to substantially increase the MFR of the modified PP which is the matrix resin. However, when the modified PP having the extremely high MFR is manufactured by means of an extruder, the strands are often cut owing to the shortage of melt tension and the strands fuse to each other, so that productivity declines.